The present invention relates generally to atomic absorption spectroscopy and more particularly to a furnace for electrothermal atomization of samples in atomic absorption spectroscopy.
Atomic absorption spectroscopy is an analytical method for determining the amount or concentration of a looked-for element in a sample. The sample is atomized such that its elements are present in atomic state in a "cloud of atoms". A measuring light beam from a line emitting light source which comprises the resonant spectral lines of the looked-for element is passed through this cloud of atoms. The concentration of the looked-for element in the sample can be determined from the attenuation of the measuring light beam in the cloud of atoms after calibration with a known sample.
Electrothermal atomization of the sample is preferable for high sensitivity measurements. In electrothermal atomization, the atomization of the sample and the generation of the cloud of atoms takes place in a electrically heated furnace. Generally, this furnace is a suitably designed graphite body referred to as a graphite tube atomizer which is heated by a high electric current. The sample is introduced into this furnace which is heated to high temperature by passing electrical current therethrough. The sample is thereby first dried, then ashed and lastly atomized. A "cloud of atoms" is accordingly generated in the furnace which contains the looked-for element in an atomic state. The measuring light beam is passed through this furnace.
Generally, these furnaces consist of a small tube made of graphite which is held between two annular contacts. A high electrical current is passed through the contacts and through the tube in its longitudinal direction. Thus, the tube can be heated to high temperatures. In operation, the sample is inserted into the tube through a lateral inlet port and is atomized when the tube is heated up. The measuring light beam passes through the annular contacts and longitudinally through the bore of the tube in its longitudinal direction. The graphite tube is surrounded by an inert gas on the inside and outside which prevents the tube from contacting air oxygen. Such graphite tube atomizers are illustrated and described in the commonly owned Schmedes et al., U.S. Pat. No. 3,778,156 issued Dec. 11, 1973 and Huber et al., U.S. Pat. No. 4,098,554 issued July 4, 1978 (both incorporated herein by reference).
In the commonly owned R. Tamm, U.S. Pat. No. 4,111,563 issued Sept. 5, 1978 (which is incorporated herein by reference), a graphite tube is shown in which a tubular inner body member is arranged in the central area within the tubular furnace body which is open at both ends with the tubular inner body member being substantially shorter than the furnace body. The inner body is arranged concentric in the furnace body and extends only through the central area of the furnace body. A lateral inlet port is centrally positioned in the furnace body in alignment with an inlet port in the tubular inner body. The inner body is connected with the furnace body through longitudinally extending webs which extend in the longitudinal plane perpendicular to the inlet port.
The furnace of Tamm reduces errors in measurement due to unhindered spreading of the liquid sample over the inner wall of the graphite tube. If the liquid reaches the relatively cool end portions of the tubular furnace, there may only be an incomplete vaporization such that sample material is retained in the furnace which will disturb subsequent measurements of other samples. Further, this configuration inhibits the infiltration of the liquid sample into the inner wall and avoids sample losses which can occur by the seeping of sample liquid into the porous graphite.
It is desirable to delay the atomization of the sample relative to the heating of the furnace wall. An adequate atomization delay ensures that the components of the atomized sample do not precipitate on relatively cool wall portions and that the sample is atomized as abruptly as possible to generate a strong absorption signal. From L'vov's publication in "Spectrochimica Acta" vol. 338, 153-193 a generally rectangular platform, made of pyrolytic graphite, is known which is inserted into a furnace designed as a graphite, tube. In order to reduce the contact with the graphite tube wall, cutouts are provided along the longitudinal edge of the platform. As a result, the sample is heated substantially indirectly by radiation of the inner wall of the furnace.
In the commonly owned Glaser et al., U.S. Pat. No. 4,303,339 issued Dec. 1, 1981 (which is incorporated herein by reference), an inner body in the shape of a platform is shown which has a recess for accommodating the sample and which is guided at the outer body only along two opposite longitudinal edges. The amount of sample which can be accommodated by this platform is limited. The handling of the platform is complicated and requires considerable skill, i.e., into a small furnace body, an even smaller platform has to be inserted which is a very complicated manipulation. Furthermore, some electrical current flowing through the furnace body in the longitudinal direction is also flowing through the platform so as to generate heat. Therefore, the platform is not only heated indirectly by radiation but also by Joul's heat generated in the platform itself.
It is a object of the present invention to provide a new and improved furnace for electrothermal atomization.
Another object of the invention is to provide such a furnace which avoids sample precipitation or collection on the interior furnace wall.
A further object of the invention is to provide such a furnace wherein the sample is heated indirectly on a platform with a predetermined delay relative to the heating of the furnace body.
Another object of the invention is to provide such a furnace wherein a sample held on a sample platform is predominantly heated by heat radiation from the furnace wall.
Another object of the invention is to provide such a furnace which avoids difficult handling and positioning of a platform and insures the platform is exactly positioned in the furnace body.
Yet another object of the invention is to provide such a furnace which is economical to manufacture.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
Accordingly, it has been found that the foregoing and related objects are attained in an electrothermal atomization furnace having a tubular electrothermal furnace body with an integral sample platform. The sample platform is hollow, generally semicylindrical shape and is integrally connected to the furnace body by a web configured and positioned to sufficiently impede heat conduction from the furnace body to the platform so that sample on the platform is thermally atomized substantially by radiation from the furnace body.
The inner body is integral with the tubular outer furnace body and therefore the furnace with the inner body can be handled easily. In contrast to U.S. Pat. No. 4,111,563, however, the inner body is not a closed tube but is a hollow, generally cylindrical shape and is open to the inner wall of the furnace body. Therefore, the sample supplied to the inner body is heated with a delay relative to the furnace body. Although the inner body in U.S. Pat. No. 4,111,563 is also heated with delay relative to the actual furnace body, the sample does not "see" the inner wall of the furnace body but rather the sample sees the inner wall of the inner body and is heated together with this inner body. Therefore, the effect which should be achieved by delayed heating of the sample does not occur in the arrangment described in U.S. Pat. No. 4,111,563. On the contrary, the same undesirable effects--now with respect to the inner body--occur which should be avoided by the platforms according to L'vov's publication and according to U.S. Pat. No. 4,303,339. In the present invention, the furnace is easily handled. The hollow, generally semicylindrical design of the inner body offers the advantage that larger amounts of sample can be dosed. The present furnace is economical to manufacture because manufacturing includes lathing, boring, and milling processes which are easily carried out.